Baryons and Dark Matter in the Galaxy
Dark matter—matter that gravitates but does not radiate—has been studied by astronomers for more than a century. Evidence for its existence comes from observations of clusters of galaxies, of the rotation curves of galaxies and of the cosmic microwave background. Its nature is unknown; we do know that it is not baryonic and that its observational effects cannot be fully explained by modifying the law of gravity. The amount of dark matter in the solar neighborhood can be inferred by comparing the baryonic mass in stars and gas with the gravitating mass inferred from stellar kinematics. A new census of the baryonic mass near the Sun gives an improved estimate for the amount of local dark matter. Remarkably, the total density of matter near the Sun is within 1% of the value estimated by Oort in 1932! It has been suggested that there is a thin disk of dark matter in order to account for terrestrial phenomena that have a periodicity of about 30 Myr, but such a disk is difficult to accommodate. The Galaxy extends out to a distance of order 250 kpc, and observations indicate that this is filled with gas with temperatures in the range a few hundred thousand to a few million degrees. Models of the gas that fit the observations show that there is enough gas that there may be no “missing baryon” problem in the Galaxy, that it contains a significant amount of heavy elements and that there is enough of it to enable the Galaxy to continue to produce stars at the current rate for tens of billions of years.